1. Field of the Invention
The invention concerns a color display device that can be used, notably in the instrument panel of a military aircraft, to give the pilot a high definition color image, and that can also be used in applications for the general public, to make television image projectors.
2. Description of the Prior Art
There is a known method, described notably in the French patent application No. 2 584 257, for making a collimated, high definition color display device comprising:
three cathode-ray tubes controlled respectively by three electrical signals representing, respectively, a red image, a green image and a blue image; PA1 a first optical mixer device having a first dichroic strip to give a beam resulting from the superimposition of a beam of rays coming from the green cathode-ray tube and a beam of rays coming from the red cathode-ray tube; PA1 a second optical mixer device having a second dichroic strip receiving the resultant beam given by the first mixer device and receiving a beam of rays coming from the blue cathode-ray tube to give a beam resulting from the superimposition of the three beams, red, green and blue respectively; PA1 an objective to collimate the resultant beam given by the second optical mixer device and to transmit it towards an observer. PA1 three liquid crystal optical valves controlled respectively by three electrical signals respectively representing a red image, a green image and a blue image; PA1 a single source of white light; PA1 means to separate the white light into three colored beams, red, green and blue respectively; PA1 means to guide the three colored beams on the three valves; PA1 a dichroic tube to superimpose the three colored beams respectively modulated by the three valves, this dichroic cube being formed by four conjoined prisms, the interfaces of which have dichroic coatings; PA1 a projection objective to form a trichromatic image on a diffusing screen, using the beam resulting from the superimposition of the three colored beams modulated by the three valves.
A device such as this enables the observer to perceive a trichromatic image collimated at infinity, or at least at a distance of some meters, to avoid the observer's having to make an effort of visual accomodation when he shifts his sight from outside the aircraft to the display device.
A known device such as this makes it possible to obtain far higher definition and far more efficient luminosity than a single mask-type trichromatic cathode-ray tube or a single trichromatic liquid crystal valve. It naturally takes up more space, but it is possible to envisage the use of a red and blue cathode-ray tube having lower definition than that of the green cathode-ray tube since the eye is less sensitive to the colors red and blue. However, in all cases, this device with two or three cathode-ray tubes has the drawback of taking up much space and, furthermore, raises many problems in superimposing the three images and maintaining this superimposition in spite of the different thermal drifts that affect the position of the images in the tubes, and affect the position of the tubes themselves.
The use of dichroic strips to superimpose the three images given respectively by the three cathode-ray tubes produces colorimetrical aberrations due to the fact that the light rays coming from the cathode-ray tubes do not all have the same incidence on the dichroic strips. For, the dichroic strips have a cut-off wavelength that depends on the angle of incidence. It is possible to consider replacing each cathode-ray tube of this known device by a liquid crystal optical valve, provided with a light box comprising a monochromatic filter and a diffusing screen. The device obtained takes up less space than the device with three cathode-ray tubes, but there remains a reduction in contrast for the light rays coming from the diffusing screen and going through the optical valve are not all orthogonal to the plane of this valve. Now, the attenuation of a light ray by a liquid crystal valve is a function of the angle of incidence of this ray. Furthermore, the colorimetrical aberrations due to the dichroic strips are identical to those of a device with several cathode-ray tubes.
In the field of color television projectors, there is a known way, described in the SID-87 DIGEST, pp. 75 to 78, for making a projection of a trichromatic image on a diffusing screen by means of an optical system comprising:
This prior art device has the advantage of being more compact than a device with two or three cathode-ray tubes, and of using only one white light source, the light emission of which is divided into three spectral bands to obtain the three colored beams. This makes it possible to obtain a given luminosity for a lower electrical consumption than with three light boxes, each provided with a filter, since each filter eliminates two-thirds of the light spectrum and hence wastes a large portion of the energy given to each light box. Furthermore, the problems that arise in superimposing the three images are reduced. For, the position of each pixel is defined by the structure of each valve instead of being defined by the aiming of an electronic beam and the valves are very close to the dichroic tube superimposing the three images.
It is possible to consider adapting this known device in order to make a collimated display device for an aircraft instrument panel. However, it is desirable to achieve the utmost possible reduction in the space occupied by it, namely in its space factor, because the device has to be inserted into a very restricted space behind the instrument panel. The space factor of the device is determined essentially by the space factor of the means used to separate the illumination white light into three colored beams and to guide these three colored beams on to the three valves.
Similarly, to make a projector of television images, for use by the general public, it is desirable to reduce the space occupied by the device, so as to make it more practical to use this device and less costly to manufacture, store and distribute it in the market.